As it is known, fax machines are connected to an external communication line for receiving and transmitting documents through it.
The data on the documents, once they have been received through the line, are printed by a printing unit in the fax machine on blank sheets, which in turn are generally extracted from a first tray containing a certain supply of such sheets.
The documents to be transmitted, also referred to as original sheets or documents, are stored in a second tray, from which they are extracted to be read by the fax machine, and thus converted into data that is successively transmitted through the line.
The tray, in which the blank sheets to be printed are contained, and the relative devices for the extraction of the sheets from the tray form a group briefly referred to as ASF, acronym of the English expression “Automatic Sheet Feeder”.
The tray, in which the original sheets of the documents to be transmitted are contained, and the relative devices for the extraction of such sheets from said tray form a group briefly referred to as ADF, acronym of the English expression “Automatic Document Feeder”.
The two trays that contain the blank sheets to be printed and the original sheets to be read, respectively, together with the relative devices adapted for the extraction of the sheets form a group that represents an important aspect of a fax machine, which influences both the performance and the cost of the fax machine to a considerable extent.
Above all it is important that this group is able to ensure highly precise, accurate and reliable extraction of the sheets from the trays one at a time, and that it is of a simple construction in order to have the least possible influence on the overall cost of the total fax machine.
A device is already known, from the U.S. Pat. No. 6,022,013 adapted for assembly on an office machine, such as a fax or printer for example, which is intended to selectively feed sheets contained in two separate trays by means of a pick-up unit positioned between the two trays.
In particular in this device, the two trays are fulcrum-mounted on a fixed structure and are activated alternatively by a motor, according to the respective rotation direction, in a manner so that they selectively approach the pick-up unit, thus causing the separation and feeding of one sheet at a time from one or the other of the two trays.
However, the Applicant has observed that this device has the drawback of having a rather complex structure, and thus expensive, and that it is furthermore conditioned by the fact that both trays need to be moved in order to bring the respective sheets in a position to cooperate with the pick-up unit.
In fact the movements to approach and move away the two trays with respect to the central pick-up unit, because of the trays mass, provoke dynamic stress that can sometimes have a negative effect on the performance and efficiency of the picking-up of the sheets, and in extreme cases, even compromise the stability of the sheets in the respective trays.
Also known through U.S. Pat. No. 6,688,590, is a printer having two trays for containing two corresponding reams of sheets of paper, and an extraction device positioned between the two trays to selectively extract sheets from one or the other of the trays.
The extraction device is composed of a drive shaft and an extraction member having an arm structure that rotates around a fixed axis, and intended to be driven by the drive shaft to determine the extraction of the sheets.
When in use, the extraction member oscillates around the fixed axis, to move between the two trays and to engage one or the other of the reams of sheets contained in the two trays, operating in a manner so that when the drive shaft rotates in a first rotation direction, the extraction member engages and extracts a sheet from a first ream, and when the drive shaft rotates in a second opposite rotation direction, the extraction member engages and extracts a sheet from the second ream of sheets.
The Applicant has observed that in said extraction device of sheets from two trays, the extraction member has a rather complex structure and furthermore, is not easily controllable because of the arm and projecting shape.
Also known through U.S. Pat. No. 6,709,177 is a device applied to a photocopying machine for extracting and feeding sheets from two trays, positioned one on top of the other, wherein each of the two trays is associated with a respective leafing roller for extracting and feeding one sheet at a time from a ream of sheets contained in the corresponding tray.
In this case as well, the Applicant has observed that the extraction and feeding device for the sheets from the two trays has the drawback of having a rather complex structure, and therefore expensive, especially because of the presence of a specific and separate feeding roller for each one of the trays.